That can also be avoided in the modeling stage, if you know before hand it will be rendered and you want a smoother curve, by setting the curve segments to a higher number.

There is a "CloseOpens.rb" ($ 20) at Smustard's that is supposed to find these little breks and close them. Also I suggest you to use "SimplifyContours.rb" (croll down to the bottom) once you have very dens survey line system. It won't really affect the appearance of your terrain but may be of big help with big and detailed areas.